The power supply of the electric motor of electrically powered or hybrid electric vehicles is a multi-cell series battery in which plural battery cells are connected in series to produce the desired driving voltage. The battery cells of the aforementioned multi-cell series battery are secondary batteries which store the electric power generated by the electric motor; the voltage of the battery cell increases as it is charged and decreases as it is discharged. However, over-charging and discharging the battery cell leads to the degradation not only of the torque characteristics of the electric motor, thereby affecting vehicle performance, but also of the service life of the battery cell itself. Consequently, there should be a cell voltage abnormality detector or cell voltage monitoring device that monitors the voltage of each battery cell so that if the cell is being over-charged or discharged this abnormality can quickly be detected and reported to the controller, etc.
In a typical cell voltage abnormality detector of the prior art, as described in Japanese Kokai Patent Application No. 2001-327091, for example, each of the battery cells that form the multi-cell series battery is connected in parallel with a cell voltage detection element, and the sum of the currents flowing in all of the cell voltage detection elements is detected by an element current detection means or current transformer. The detected total current value is then compared with a reference value by means of comparison means, which determines whether battery cell abnormalities have taken place.
A schematic of the device used to detect or measure the voltage of the various battery cells of a multi-cell series battery shows the function of detecting cell voltage abnormalities (over-charging and discharging). For example, the cell voltage detector described in Japanese Kokai Patent Application No. 2001-20152 includes a cell selection switch, a sampling switch for sampling the voltage of the selected battery cell, a capacitor that is charged by the sampled voltage, and a transfer switch that transfers the sampled voltage to an A/D converter after the sampling switch is turned off. By comparing the measured digital current value obtained from the output of the A/D converter with a reference value, it can detect an abnormality (over-charging or discharging).
However, the cell voltage abnormality detector described in Japanese Kokai Patent Application No. 2001-327091 has a complex circuit constitution since each battery cell has a cell voltage detection element and is large and costly since an element current detection means (current transformer) is required to detect the sum of the currents flowing in all of the cell voltage detection elements. Moreover, random cells cannot be identified and checked, which is inconvenient and limits the utility of the detector.
The cell voltage detector described in Japanese Kokai Patent Application No. 2001-20152, on the other hand, is time-intensive since before the state of the battery cell selected from the multi-cell series battery by the cell selection switch can be determined, time is required for charging the capacitor via the sampling switch, for pulling down the potential of the capacitor to ground via the transfer switch and then for transfer to the A/D converter, and also for A/D conversion by the A/D converter. Consequently, abnormality detection is delayed, but a delay in detecting a battery abnormality in an electric vehicle, etc., can be fatal to safe battery operation. Also, in this case, the constitution of the voltage detector (particularly the sampling switch, capacitor and transfer switch) and its operation (switching operation, charging/discharging operation) is complicated. Moreover, when an abnormality of the measured cell voltage value is found, there is no way to determine whether it is actually a cell voltage abnormality or a problem with the measurement circuit, which is also undesirable.